Heretofore, a circuit board has been one having an electrically conductive metal circuit joined to a main surface of a ceramic substrate on which a semiconductor is to be mounted by means of solder, and having a semiconductor device mounted at a predetermined position on the metal circuit. In order to maintain high reliability of a circuit board, it is required that heat generated from a semiconductor device should be dissipated so that the temperature of the semiconductor device will not become excessively high, and thus a ceramic substrate is required to have excellent heat dissipation properties in addition to electrical insulating properties. In recent years, along with miniaturizing of a circuit board and high output of a power module, for a small light-weight module, a ceramic substrate employing a silicon nitride (hereinafter referred to as “SN”) sintered body with excellent mechanical properties having high electrical insulating properties and high thermal conductivity and a ceramic circuit board having a metal circuit formed on a main surface of an SN substrate have attracted attention.
A ceramic sintered body to be employed for a ceramic substrate is produced usually by the following process. Namely, a ceramic powder is mixed with additives such as a sintering aid, an organic binder, a plasticizer, a dispersant and a release agent in a suitable amount, and the mixture is molded into a sheet by extrusion or tape molding. Then, the molded product is heated in the air or in an inert gas atmosphere, e.g. nitrogen, at from 450 to 650° C. to remove the organic binder (debindering step), and heated in a non-oxidizing atmosphere such as nitrogen to produce a sintered body. In the case of an SN sintered body, the molded product is pressed in a non-oxidizing atmosphere such as nitrogen and held at from 1,600 to 1,900° C. for from 0.5 to 10 hours (firing step) to produce a sintered body.
In general, by employing extrusion, the thickness in molding will not be limited any more and it is possible to mold a ceramic sheet in the form of either a thin plate or a thick plate. First, a powder mixture comprising a ceramic powder, a sintering aid and an organic powder binder is prepared by using e.g. a universal mixer, an automatic mortar, a mixer or a vibrating sieve. The powder mixture is sprayed with a liquid mixture comprising e.g. water, a release agent and a plasticizer to prepare a granular wet powder material (granulation step) by using e.g. a universal mixer, an automatic mortar, a mixer or a vibrating sieve. Then, this wet powder material is charged into a material feed opening of a kneader to prepare a kneaded clay (kneading step). The kneaded clay thus prepared is charged into a material feed opening of a single screw extruder provided with a die and molded into a sheet. (Patent Documents 1 and 2).
Patent Document 1: JP-A-2-83265
Patent Document 2: JP-A-11-21174